So what is this stuff?
If you’ve ever gotten a soil test, you’ve probably seen that one of the tests that can be conducted is “percent organic matter.” What does this mean, and what does it have to do with growing crops? To start, we first have to understand what soil is. Soil is a mix of materials that we can broadly categorize into inorganic and organic compounds. The inorganic material is comprised of fine, granular pieces of rock we call sand and very small particles we call fines. Fines may be categorized as clay or silt. What’s left is soil organic matter, which is made of small plant residues, actively decomposing residues and microorganisms, and the residues that have finished decomposing, which is called “humus.”
Soil organic material benefits crop land in three main ways: water and nutrient retention, soil structure improvement, and microbe management. Microbe management is simple enough to understand. Beneficial soil microbes need something to feed off in order to maintain a healthy population, and a steady supply of fresh organic matter is imperative to their survival. Soil microbe management is especially important in USDA certified organic farming operations, as soil microbes are often necessary to break down organic fertilizers into components that are most efficiently utilized by crops.
Soil structure improvement and the water and nutrient retention qualities of organic material comes down to their physical properties. Organic materials have a larger surface area than sand, which means it has more binding sites for water and nutrients. Clay and silt fines also have a large surface area and can hold much more water than sand grains, but unlike organic matter are less willing to give water to roots. A soil with a higher organic content will therefore be more efficient than a lower organic content in holding onto nutrients and water long enough for plants to take them up before they sink below the root zone.
Organic matter tends to bind to itself better than surrounding particles, a characteristic called aggregation. The spaces between organic matter and surrounding particles forms channels for water to penetrate, so soils with well-dispersed organics allow for good water penetration. They also resist breakup and movement, which improves soil stability.
What should my organic percentage be?
The perfect agricultural soil is, for most crops, well-draining but still able to hold onto water for a little bit. You want rain or irrigation water to drain of course, but not before the plants are able to take up what they need. However, the increased water-holding capacity ideally comes not from increasing our percentage of fines but from organic material such as a good compost that is worked into the soil before planting. These organics are good at holding water but also provide binding sites for nutrients and enhance the microbial ecosystem, which is beneficial to plant health and absolutely essential if you even think you want to do organic farming. While a few crops do thrive on nutrient-poor soils, for most crops we’d like to be somewhere between 3-6% organic matter content in the soil.
Once you have a good organic content in your soil, you’ll need to replenish it each year with compost and/or cover crops, which are plants that we put in primarily for the benefit of maintaining soil health and not for commercial harvest. They are especially important for farms that do not or are unable to use no-till practices. These cover crops become organic material as they break down, and some provide other benefits such as nitrogen fixation, pest reduction, and soil erosion prevention. Soil organic matter is a major component in the construction of healthy farmland.